In the field of ionics, there is a trend of down-sizing as well as making it to be of the solid-state type, and efforts are being made extensively with view to application, to solid-state primary or secondary batteries and electric double layer capacitors, of solid electrolytes as a new ionic conductor which replaces conventional electrolyte solutions. Conventional batteries with electrolyte solutions have problems in long-term reliability since there tends to occur leakage of the electrolyte solution out of the parts or elution of the electrode substance. On the contrary, products with solid electrolytes do not cause such problems and it is easy to make their thickness smaller. Furthermore, solid electrolytes are excellent in thermal resistance and advantageous in the manufacturing process of products such as battery.
Among batteries using a solid electrolyte, those using a polymer as a main component of the electrolyte have a merit of increased flexibility as compared with those using an inorganic substance, which endows the former with processability into various forms. However, such products as hitherto studied still suffer from a problem that only a small amount of current can be taken out since the solid polymer electrolyte has a low ionic conductivity.
As an example of such solid polymer electrolyte, it is described in British Polymer Journal (Br. Polym. J.), 7: 319-327 (1975) that a compounded material consisting of a polyethylene oxide and an inorganic alkali metal salt exhibits ionic conductivity, which is, however, as low as 10.sup.-7 S/cm at room temperature.
Recently, there have been many reports that a comb-shaped polymer having an oligooxyethylene in its each side chain has an improved ionic conductivity due to increased thermal motion of the oxyethylene chain which contributes ionic conductivity. An example in which polymethacrylic acid with oligooxyethylene being added to its side chain is compounded with an alkali metal salt is described in Journal of Physical Chemistry (J. Phys. Chem.), vol. 89, page 987 (1985). Another example in which polyphosphazene with an oligooxyethylene side chain is compounded with an alkali metal salt is described in Journal of American Chemical Society (J. Am. Chem. Soc.), vol. 106, page 6854 (1984).
Recently, many studies have been made on lithium secondary batteries in which metal oxides or metal sulfides such as LiCoO.sub.2, LiNiO.sub.2, LiMnO.sub.2, MOS.sub.2 and the like are used as positive electrode. For example, batteries using positive electrode made of MnO2 or NiO2 are reported in Journal of Electrochemical Society (J. Electrochem. Soc.), vol. 138 (No. 3), page 665 (1991). These batteries have drawn attention since they have high gravimetric or volumetric capacity.
Also, many reports have been made on batteries using electroconductive polymer as electroactive material. For example, lithium secondary battery using polyanilines as positive electrode has already been put on the market in the form of a coin type battery for use as a backup source by Bridgestone Co., Ltd. and Seiko Co., Ltd. as reported in, for example, "The 27th Symposium on Battery 3A05L and 3A06L" (1986). Polyaniline also attracts attention as an electroactive material for positive electrode having a high capacity and flexibility.
Lately, electric double layer capacitors which comprise polarizable electrodes made of a carbon material having a large specific area such as activated carbon, carbon black or the like and an ionic conducting solution arranged between the electrodes have been widely used for a memory backup source. For Example, a capacitor having carbon-based polarizable electrodes and an organic electrolyte solution is described in "Kinou Zairyo", February 1989, page 33. An electric double layer capacitor using an aqueous sulfuric acid solution is described in "The 173rd Electrochemical Society Meeting Atlanta Ga.", May, No. 18 (1988). Also, a capacitor using highly electroconductive Rb.sub.2 Cu.sub.3 I.sub.3 Cl.sub.7 as an inorganic solid electrolyte is disclosed in Japanese Patent Application Laid-open No. 63-244570 (1988).
However, electric double layer capacitors using conventional electrolyte solutions have problems in long-term use and reliability since there tends to occur leakage of the solution out of the capacitor when used for a long time or when a high voltage is applied to. On the other hand, electric double layer capacitors using conventional inorganic based ionic conducting substances have a problem that the ionic conducting substance decomposes at a low voltage and, hence, the output voltage is low.
The use of an ionic conducting substance using a polyphosphazene based polymer for a capacitor is disclosed in Japanese Patent Application Laid-open No. 4-253771 (1992). The capacitor using such an ionic conducting substance containing a polymer as the main component has various merits that it has an output voltage higher than the inorganic substance based ionic conducting substance, can be processed into various forms and is easy to be sealed.
In this case, however, the ionic conductivity of the solid polymer electrolyte as low as 10.sup.-4 to 10.sup.-6 S/cm is still unsatisfactory and there is also a defect that the take-out current is small. Although it is possible to increase the ionic conductivity of the solid polymer electrolyte by the addition of a plasticizer, this gives fluidity to the electrolyte, which means that the electrolyte can no longer be treated as a complete solid. As a result, the film made of the polymer has a poor film strength and the polymer has a poor film formability and, hence, not only short-circuit tends to occur when the polymer is used in an electric double layer capacitor or a battery but also there arises a difficulty in sealing as in the case of the liquid ionic conducting substances. In addition, when it is assembled with a polarizable electrode such as a carbon material to produce a capacitor, there arises a problem of homogeneously compounding the solid polymer electrolyte with a carbon material having a large specific surface area since the both materials are solid and hard to be homogeneously compounded.
The ionic conductivity of solid polymer electrolytes studied so far has been improved up to about 10.sup.-4 to 10.sup.-5 S/cm, which is, however, by over two digits lower than that of the liquid ionic conducting substances. Besides, at a temperature no higher than 0.degree. C., the ionic conductivity decreases drastically to a further lower level. Furthermore, when these solid electrolytes are incorporated in a device such as an electric double layer capacitor or incorporated in a battery in the form of a thin film, there arises difficulties in the manufacturing process since the solid electrolyte is hard to be compounded with the electrode and it is difficult to achieve a satisfactory contact between the electrolyte and the electrode.